(1) Field of the Invention
This invention relates generally to both apparatus and methods for determining the range of a radiating source relative to an observer.
(2) Description of the Prior Art
Range finding is a process whereby the distance between an object and an observer is determined. In general, the object may be a radiating source or may be passive (i.e., an energy reflector). Of particular interest herein is the case where the object is a passive source. Either a passive object or a radiating source may be referred to as a target.
Range finding systems are either active or passive. Active systems typically involve the transmission and reception of electromagnetic energy (radar) or acoustic energy (sonar). A disadvantage of such active range finding systems is that the transmission of such electromagnetic and acoustic energy discloses the position or location of the observing ship that transmits the ranging energy.
Passive sonar merely receives the acoustic energy generated from a distant source and is only capable of giving information on the target bearing and the acoustic frequency generated by the target.
An active sensor system, such as active sonar, can determine the distance to an object directly by forming the product of (1) one half the sound velocity and (2) the time interval from the instant of the transmission of a ping until the instant of the arrival of the ping's echo from the object.
A passive range finding system, such as passive sonar, can determine the direction of a target (radiating source) and the frequency of the radiation. Current techniques of passive ranging require that (1) special conditions pertain or (2) additional information, other than frequency and bearing data supplied by passive sonar, be known or that strong assumptions be made.
Radio direction finding is another type of passive system which determines only the bearing of a source of radio or electromagnetic emission. This is usually accomplished by means of a directive receiving antenna.
There are several other techniques which may be applicable under certain circumstances when the range of an acoustic radiating source is to be determined. Bottom bounce ranging can be used when the bottom depth, gradient and composition are known. Multipath ranging can be used if the temperature profile, at the ocean location in question, is suitable.
Bottom bounce techniques require that (1) the combination of ocean temperature profile, bottom depth and composition, and target range allow reception of such a bounce and, (2) the bottom depth and orientation vector be known in advance with reasonable accuracy. Often a tentative range is calculated from a bottom bounce when the depth is known and a horizontal bottom has been assumed.
Multipath ranging uses signals received from the source by reflection from the surface, reflection from the bottom, and direct transmission. Range is calculated from the delay time between receipt of the signals using the temperature profile, and bottom and surface conditions.
Various prior art range finding apparatus are described in the following documents. In Snowden, U.S. Pat. No. 3,304,409, Feb. 14, 1967, range is determined as a function of the bearing .theta. of the target from an observing ship, and measurements of the motion of the observing ship. In Olsen, U.S. Pat. No. 3,947,804, Mar. 30, 1976, there is disclosed a range/bearing computer that solves two trigonometric equations which are indicative of the bearing and range of a target or radiating source. The variables used in the equations are the time interval between the arrivals of a wave front at different spaced locations, a constant voltage proportional to the distance of separation of the spaced locations, the velocity of propagation of the wave front, and the angle of inclination formed by the intersection of a line drawn through the spaced locations and a horizontal plane through one of the spaced locations.
None of the known methods or apparatus can directly determine the range to the object of interest, the course and speed or track of the object, or predict the position of the object at a given instant of time without reference to surface or bottom conditions.